Gabapentin, 1-(aminomethyl) cyclohexane acetic acid, is a structural analogue of the neurotransmitter gamma-aminobutyric acid. The oral absorption of gabapentin is dose-dependent due to a saturable L-amino acid transport mechanism in the intestine. Thus, the oral bioavailability varies inversely with dose. Following a dosing regimen of 900, 1200, 2400, 3600 and 4800 mg/day given in 3 divided doses, the bioavailability of gabapentin is approximately 60%, 47%, 33% and 27% respectively. Plasma concentrations are proportional with dose up to 1800 mg daily and then plateau at approximately 3600 mg daily.
Peroral administration of gabapentin to treat pain, for example in acute post-procedural pain relief, has been documented by various clinical studies. However, the peroral route has disadvantages, including uncertainty for use as pre-procedural medication. For example, gabapentin has a dose dependent extent of bio-availability, and oral absorption may be impaired because of loss of gastrointestinal function or restrictions on oral intake. For example, oral administration of gabapentin yields lower plasma concentrations because of its low bioavailability.
Accordingly, there is a need for an injectable pharmaceutical formulation comprising gabapentin or a derivative of gamma-aminobutyric acid for treating pain in general, including but not limited to post-procedural pain.